There is widely known a distributed feedback semiconductor laser which causes its active layer to generate stimulated emissions by light distribution feedback with a diffraction grating formed close to the active layer. Since the distributed feedback semiconductor laser relatively easily provides stimulated emission with excellent lasing spectral characteristics, and the lasing wavelength can be controlled by the pitch of the diffraction grating, its use is promising as the light source of long-distance and large-capacity optical communication systems and other opto-electronic devices which either utilize single mode optical fibers or wavelength division multiplexing.
Light distributed feedback in the conventional lasers is given by forming a transparent light guiding layer proximal to an active layer, forming corrugated shapes having a cross section substantially similar to triangular waves on the surface thereof other than the side near the active layer, and changing the apparent refractive index of the guiding layer periodically. The structure is popularly known and is described, for instance, in Denshi-Joho-Tsushin Handbook, pp. 984-985, a general handbook published by Ohm Sha, Japan 1988. The semiconductor laser of this structure, however, cannot conduct adequate feedback in optical phase with respect to light of Bragg wavelength generated corresponding to the period of changes in layer thickness in the light guiding layer to thereby cause stop bands in the zone of Bragg wavelength. FIG. 1 shows this phenomenon.
More specifically, FIG. 1 is a graph which plots lasing wavelength in normalized value on the horizontal axis and spectral intensity in relative value on the vertical axis. The graph shows that there is a phenomenon which causes two isolated longitudinal mode lasings at two wavelengths substantially symmetrical on both sides of the Bragg wavelength. There has been empirically, known from various tests that it is difficult to design and manufacture practical semiconductor lasers in a manner to cause either one of the two longitudinal mode lasings or to preset either one of them. Production yield cannot be increased due to the above reason.
In order to solve the problem, there has been proposed a structure which shifts the diffraction grating in phase by one quarter of a wavelength at about the center thereof. This increases the difference in gain between two longitudinal modes at two wavelengths to thereby enable setting the lasing mode at one. But the structure requires a special manufacturing process because of the complicated form of diffraction grating. The structure is further detrimental as it needs anti-reflection coating on the facet of a laser diode to thereby increase the number of manufacturing steps and also the production cost. The semiconductor laser of this structure is also described in the above mentioned handbook.
"Coupled-Wave Theory of Distributed Feedback Lasers" by Kogelnik et al., Journal of Applied Physics, 1972, vol. 43, pp. 2327-2335 presents the following basic theory: although a stop band is created in the Bragg wavelength zone when light distributed feedback is given by index coupling as above, if the light distributed feedback is conducted by gain coupling based on periodic perturbation of gain coefficient longitudinal mode, then lasing of completely single wavelength should be obtained without generating any stop band. The paper is a theoretical one and gives no description on the structure of semiconductor lasers nor the manufacturing method to embody such gain coupling.
Some of the present inventors filed in Japan on July 30, 1988, a patent application (Sho 63-189593) for a novel semiconductor laser, which applies that basic theory of Kogelnik et al. (referred to hereinafter as a "prior application").
The technology of the prior application teaches the provision of a non-transparent semiconductor layer near an active layer, of diffraction grating on the non-transparent layer and distributed feedback based on periodic perturbation of loss coefficient of the non-transparent layer.
The above structure embodies the device which satisfies the theory of Kogelnik et al., but as this method forms an non-transparent layer near the active layer for feedback, and as the non-transparent layer has absorption loss of energy, the energy required for generation of stimulated emission become inconveniently large.
It would be optimal to form a diffraction grating on one of the surfaces of the active layer and to change the thickness of the layer in accordance with the corrugation of the diffraction grating in the direction of the light waves in order to give distributed feedback in a manner to cause periodic perturbation of gain coefficients based on the above theory. A paper by Nakamura et al. entitled "GaAs-GaAlAs Doublehetero Structure Distributed Feedback Diode Lasers", Applied Physics Letters, 1974, vol. 25, pp. 487-488, reports on the result of experiment to etch diffraction grating directly on the active layer of a semiconductor laser, although it was not for the purpose of realizing gain coupling. However, when the active layer is etched to have a corrugation as diffraction grating, a series of processes for forming the corrugation, i.e., suspending the growth, etching the layer, and resuming the growth, will cause defects in the semiconductor crystal in the layer. The defect in the semiconductor crystal increases non-light-emitting recombination to reduce the stimulating emission remarkably, thus producing semiconductor laser of inferior efficiency which is not practically usable.
This invention eliminates such defects encountered in the prior art, and realizes a semiconductor laser which can give light distributed feedback by gain coupling mainly based on the periodic perturbation of gain coefficients in accordance with the theory by Kogelnik et. al. instead of index coupling which tends to entail formation of stop bands. This invention also embodies a semiconductor laser without loss in energy absorption as is caused in the non-transparent layer in the prior application, and further, without defects in the semiconductor crystal structure even if the active layer is directly formed with diffraction grating.
More particularly, this invention provides a semiconductor laser which has a stable and single lasing mode instead of two lasing modes and which is capable of setting the single lasing mode in advance. The inventive laser is simple in structure as is its manufacturing process, has an excellent production yield, is low in cost, has no loss in energy absorption unlike the prior application laser, is free of defects in semiconductor crystal structure which is to become an active layer even if diffraction grating is formed thereon, and can efficiently produce stimulated emission. Its manufacturing method is also efficient.